1. Field of the Invention
The invention pertains to apparatus and methods for detecting radiation, and more particularly to radiation detectors that rely on measuring the time between detection events to obtain useful information.
2. Description of Related Art
Radiation is typically measured by detecting an electrical pulse that is generated through an interaction or set of interactions, and is proportional to the energy deposited in the detector by the radiation. The time between individual events gives an indication of the size or strength of the radiation source. In addition, if the time between sequential events from multiple detectors is short enough, the geometry of the detectors and the energy deposited in each detector may be used to identify the approximate origin of the radiation source through a method known as Compton scattering; the device is commonly referred to as a Compton camera. Fundamental to performing this measurement, one must know the exact time between measurements in order to determine if the individual events are from a true Compton scattering interaction.
The measurement of this time difference can be performed in two different ways. The first method is to use one of the detectors to generate a start signal and the second detector to generate a stop signal. These signals trigger a stop watch function, which generates an analog signal that is proportional to the time between the signals. If the time is below a specified threshold, then the events are recorded as being coincident, or are indicated as being a Compton scattered event. This method is only practical for a small number of detectors, usually limited to only two, and does not scale well to larger systems. (The general approach is described by T. J. Paulus in “Timing electronics and fast timing methods with scintillation detectors,” IEEE Trans. on Nucl. Sci. NS-32: 1242-49 (1985). See FIGS. 12 and 13 therein for examples of system implementations. Some commercial systems and methods are described in an application note from Canberra Industries, “Timing and Coincidence Counting Systems.” In the systems taught therein, each separate box shown on the diagram is a separate instrument.)
The second method of measuring the time difference is by generating a time stamp for each event. These time stamps can then be compared to find coincident events. However, this system is limited by the accuracy and precision of the time synchronization of the individual detector electronics that generate the time stamps. While this method is adaptable for large systems, it must be designed for the specific system and is not easily scalable. Designers of several large physics experiments have independently developed methods of generating the timestamps and for distributing the clock signals in order to synchronize the units, showing that the technology is not scalable or portable. An example system includes the IceCube project based on the AMANDA clocking system, which synchronizes nodes through the distribution of a GPS time signal. For this to work each cable must be exactly the same length with a balanced tree configuration to maintain signal propagation time delays between the clock source and every node, as described in “Clock Distribution and Synchronizing to UT” by G. T. Przhybykski, Lawrence Berkeley Laboratory (2001). The STAR trigger system uses proprietary electronics to distribute the clock to each readout card located at each node of the system [“Trigger/clock distribution tree requirement document” by V. Lindenstruth et al. (1996)]. An example PET/CT system requires a time resolution of 5-4 ns and utilizes a design that propagates a low frequency clock signal between individual boards that is converted to a high speed clock using an LVPECL and propagated to the ADC and FPGA for each detector, as described in “A Data Acquisition Sub-System for Distributed, Digital, Computational, APD-Based, Bimodal PET/CT Architecture for Small Animal Imaging” by R. Fontaine et al. document No. 0-7803-8257-9/04/IEEE (2004). Standardized solutions are available from manufacturers such as the CAEN Waveform Digitizers for high density digitization and synchronization of up to 64 channels. High accuracy clock synchronization is possible in this system since all of the digitization and processing is done on the same card or circuit board for all of the channels, thus requiring all of the detectors to be located in close proximity to the digitizer. This can be problematic as most radiation induced events have low signal levels which can be easily dominated by noise.
The IEEE 1588 standard refers to a communication protocol originally developed to allow factory automation tools to communicate with one another. Some applications of IEEE 1588 protocols to factory automation are disclosed in U.S. Pat. No. 6,804,580 by Stoddard et al.; U.S. Pat. No. 7,656,751 by Rischar et al.; and U.S. Pat. No. 7,607,166 by Coley et al. It has also been applied to automated test systems, as taught for example in U.S. Pat. No. 7,707,000 by Baney et al.; and U.S. Pat. No. 7,561,598 by Stratton et al. Use in synchronizing audio devices is disclosed in U.S. Pat. No. 7,680,154 by Stanton et al. A number of patents have been issued for applications involving time synchronization in various communication networks, of which the following are typical: U.S. Pat. No. 7,689,854 to Ilnicki et al.; U.S. Pat. No. 7,630,736 to Wang; U.S. Pat. No. 7,630,728 to Cutler et al.; U.S. Pat. No. 7,486,681 to Weber; and U.S. Pat. No. 7,411,937 to Guilford.
For background purposes, the following industry standards are incorporated herein by reference in their entirety:
1. IEEE 1588-2002—original standard also known as version 1, IEEE 1588v1, or PTPv1
2. IEEE 1588-2008—revised standard also known as version 2, IEEE 1588v2, or PTPv2
3. Synchronous Ethernet (SyncE) Specification—ITU-T Rec. G.8261 (http://www.itu.int/rec/T-REC-G.8261-200804-l/en)
4. SyncE Slave Clock Specification—ITU-T Rec. G.8262 (http://www.itu.int/rec/T-REC-G.8262/en)
5. SyncE Related Specification—ITU-T Rec. G.8264 (http://www.itu.int/rec/T-REC-G.8264-200810-l/en)